Legionella pneumophila, the etiological agent of Legionnaires' disease is an intracellular pathogen. Essential to its pathogenicity is the Dot/Icm type IV secretion system that translocates a large number of bacterial effectors into host cell. These effectors modulate multiple host cell processes to create an endoplasmic reticulum (ER) derived organelle permissive for Legionella intracellular growth. We recently found that one of such effectors called SidJ plays an important role in the biogenesis of replicative Legionella-containing vacuoles (LCVs) and loss of SidJ results in a temporal delay in the recruitment of ER proteins to the LCVs. Further studies indicate that SidJ is a protein phosphatase that interacts with and dephosphorylates Sec31, the major subunit of the outer shell complexes of the COPII-coated vesicles. Our working hypothesis is that SidJ, by coordinating with other effectors, redirects the trafficking of COPII-coated vesicles to the bacterial phagosomes. The goal of this project is to characterize SidJ and its protein phosphatase activity in Legionella infection using genetic, biochemical and cell biological approaches. We propose to: 1) Analyzing the interactions between SidJ and Sec31 and the effect of SidJ on host secretory pathways. 2) Studying the functional consequence of SidJ-Sec31 interaction on COPII coated vesicle formation. 3) Identification and characterization of effectors that function synergistically with SidJ. Results from this study will contribute to the emerging field of cellular microbiology and to the field of cell biology, particularly the roles of protein phosphorylation of COPII components in the regulation of vesicle trafficking. These studies will also expand our research capacity, thus allowing us to establish a research program with a broader spectrum of projects. A K02 award would allow me to invest more effort to develop my research career by exploring and acquiring background knowledge and new techniques necessary for future success in this multidisciplinary field. Relevance: Because of its potential fatality, infection by Legionella pneumophila is a concern of public health. Our study can provide novel strategies for its prevention and/or treatment. Furthermore, understanding of mammalian cell processes affected by this bacterium could lead to clues in our study of other diseases associated with abnormality of these processes.